I finally resolved this problem by dismantling the laptop and cleaning it. The finned heat exchanger at the end of the heat pipe transfering heat to the fans was severely clogged with dust. Even one of the fans had stopped working because of the quantity of dust buildup.
It is a delicate operation but well worth the effort. My laptop is now back to its normal performance. The screen, touchpad and keyboard were the three elements that needed care in handling.